1. Field of the Invention
The present invention relates to a liquid crystal display device having a ferroelectric liquid crystal layer, particularly to an liquid cyrstal display device having a fine pattern of fine pixel pitches and narrow gaps between pixels and its driving method.
2. Prior Art
Ferroelectric liquid crystal has been attracted attention as a new liquid crystal device which has spontaneous polarization with a presponse time of a microsecond order with a memory feature in a thin layer cell. The configuration of the conventional ferroelectric liquid crystal element is that a ferroelectric liquid crystal is sandwiched between two substrates each having a pattern of stripe-shaped transparent electrodes formed on a glass and two an alignment layer formed on the electrode pattern. The substrates are disposed such that the electrode patterns are in a perpendicular relation to each other. The alignment layer is a high polymer having been rubbed or an obliquely deposited inorganic material layer such as SiO. The Chiral Smectic C phase as a typical ferroelectric liquid crystal has a layer structure. In case the liquid crystal layer is thick, the molecules take a spiral stucture in a normal line direction to the layer. But in case the liquid crystal layer is thin, the spiral is loosened and thus several conditions become stable in which the molecules are uniformly directed in one direction due to the interaction with a substrate surface. An electric field allows switching between these conditions, and as this switch shows an abrupt threshold, the matrix drive is feasible. As the stable conditions, there are two uniform states in each of which spontaneous polarizations are uniformly oriented in the direction of the electric field, and as another state, there is a twist state in which the directions of the spontaneous polarizations are opposite at opposite surfaces and has a twisted structure of liquid crystal molecules therebetween. In the uniform state, liquid crystal molecules are nearly parallel with the substrate surface and oriented in different directions by 2 times the inclined angle from the normal line to the liquid crystal layer depending upon the direction of the electric field. Sandwiching a cell between two polarizers which are arranged orthogonally to each other and also aligning the polarization axis to the molecule direction of one of the two uniform states provide two display states of brightness and darkness between the uniform states. The twist state is induced by an intermediate voltage of the voltage necessary to switch between the two uniform states, and it becomes a gray indication in the about arrangement of the polarizers. (N. A. Clark, S. T. Largerwall, "Submicro second bistable electrooptic switching in liquid crystals", Applied Physics Letters, Vol. 36 (11), pp. 899-901, Jun. 1, 1980).
The stability of these stable conditions depends on the polarities of the upper and lower substrates, and the initial molecular orientations immediately after the ferroelectric liquid crystal was sandwiched in a symmetrically structured cell contain two uniform states in a mixed mode or take a twist state. So far the liquid crystal condition outside the pixel has been considered that it is kept as the initial molecular orientation and is not responsive to the electrical field.
In case of the ferroelectic liquid crystal, it was reported that the orientated domain is easily expandable in a two dimension but the expansion due to the move of the domain wall will be as large as about 1 .mu.m. (N. A. Clark, S. T. Lagerwall: "Structures and Applications of SSFLC Devices", Digest of JAPAN DISPLAY'86, pp456). Therefore, since the contrast can not be raised when a bright area display is present in the gap between pixels even if brightness and darkness is provided by controlling the voltage in the pixel area, a light shielding layer, namely, a black matrix layer, in the gap between pixels is normally formed in order to improve the contrastratio. Especially when a pixel pitch becomes smaller, several 10 .mu.m, a light shielding layer is essential in order to obtain an indication of better contrast because an opening ratio (a ratio of the pixel area to the whole display area) becomes smaller.
However, when the pixel pattern becomes smaller, it becomes difficult to form a light shielding layer precisely between the pixels and the cost thereof becomes higher. In addition, as it is necessary to make the width of the light shielding layer larger than the gap between pixels by considering the margin for processing, the opening ratio becomes smaller to result in image a dark indication. (T. Leroux et al., "BLACK AND WHITE FLC TV PANEL WITH GREY LEVELS", CONFERENCE RECORD of 1988 INTERNATIONAL DISPLAY RESEARCH CONFERENCES, pp111, 1988), (Matsumoto et al.: "Large-Area Video-rate Multicolor Ferroelectric Liquid-Crystal Display", SID 88 DIGEST, pp41-44)
As a means other than the light shielding layer formation, it is proposed to darken the gap between pixels at the initial molecular orientation by first making the polarities of the alignment layers different from each other on the upper and the lower substrates to enhance the stability of one of the two uniform states. (for instance, Japanese Patent publication 63-225224, Japanese Patent publication 63-228130). However, with this method, such an unfavorable influence is apt to occur that molecules are liable to respond to cross-talk at multiplex driving due to the drop of stability of the other uniform state to cause reduction of contrast and change in memory state. In addition, it is difficult in the forming technique to change the kind of the base of the light shielding layer or the light shielding layer itself at only the gap between pixels.
There have been such problems as mentioned above to obtain a high contrast image on a ferroelectric liquid crystal panel, especially on a panel having fine pixel patterns.
Further, there is a problem of difficulty to make a multi-gray scale because the ferroelectric liquid crystal can have only limited stable states. An intermediate state between the two uniform states becomes a white/black spotted pattern or a twisted state, thus exhibiting an intermediate gray scale condition. But, extremely high niformity of the liquid crystal panel is reguired to realize, a uniform intermediate gray scale image. Therefore, conventional surface-stabilized ferroelectric liquid crystal provides gray scale with a bi-level display by scanning plural pixels plural times. (T. Leroux et al., "BLACK AND WHITE FLC TV PANEL WITH GREY LEVELS", CONFERENCE RECORD of 1988 INTERNATIONAL DISPLAY RESEARCH CONFERENCES, pp111-113, 1988). However, since the use of plural pixels to indicate the gray scale level will increase the number of electrodes, the electrode pattern will become finer and the number of driving circuits will also increase, thus leading to increase in cost.